There has been known a variable hydraulic motor driving device capable of switching an operation state of a hydraulic motor while driving the motor, between the low-speed mode of low-speed rotation and the high-speed mode of high-speed rotation (see patent documents 1 and 2). A device disclosed in the patent document 1 (speed change device of hydraulic traveling vehicle) is mounted in a hydraulic traveling vehicle of which speed can be changed between a low-speed and a high-speed. This device enables an operator to perform change operation between low-speed travel and high-speed travel, depending on the conditions of a site or a purpose. Thus, that device is configured so that: when a speed selecting switch (13) is set to OFF, the hydraulic motor rotates at low speed; while when the speed selecting switch (13) is set to ON, the hydraulic motor rotates at high speed.
On the other hand, a device disclosed in the patent document 2 (speed change device of hydraulic traveling vehicle) was made for the purpose of eliminating the inconvenience of the above-described change operation. The device disclosed in the patent document 2 has a circuitry configured so that the travel mode can be changed simply by setting the speed selecting switch (13) to ON: the travel mode is switched to low-speed travel at a time of start and immediately before a stoppage, is switched to low-speed travel when a traveling load becomes large during high-speed travel, and is returned to high-speed travel when the traveling load is decreased.
Patent document 1: Japanese Unexamined Patent Publication No. 235135/1988 (Tokukai 63-235135) (pages 1 to 2, and FIG. 2)
Patent document 2: Japanese Unexamined Patent Publication No. 235135/1988 (Tokukai 63-235135) (pages 3 to 4, and FIG. 1)